LCDs have been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), and video cameras, due to advantages such as portability, low power consumption, and low radiation. LCDs are poised to completely replace cathode ray tube monitors and televisions in some markets. A typical LCD includes an LCD panel, a backlight for illuminating the LCD panel, and a backlight control circuit for controlling the backlight. When a cold cathode fluorescent lamp (CCFL) is employed as the backlight, a high frequency alternating current (AC) voltage is generated by the backlight control circuit for driving the CCFL.
Referring to FIG. 4, one such backlight control circuit 100 includes a control circuit 110, a transformer 120, a lamp 130, and a capacitor 140.
The transformer 120 includes a primary winding 122 and a secondary winding 124. Two terminals of the primary winding 122 are electrically coupled to the control circuit 110. One terminal of the secondary winding 124 is grounded via the lamp 130, and the other terminal of the secondary winding 124 is grounded via the capacitor 140. The lamp 130 is a CCFL.
The control circuit 110 and the transformer 120 constitute an inverter circuit configured for providing an AC voltage to driving the lamp 130. Normally, because the AC voltage outputted from the secondary winding 124 is not a sine wave, the capacitor 140 and the secondary winding 124 need to form an resistor inductor capacitor (RLC) resonant circuit in order to provide an AC voltage with a desired sine wave for driving the lamp 130.
The RLC resonant circuit includes a fixed resonant frequency f0. When the resonant frequency f0 is equal to or close to a driving frequency of the AC voltage, an efficiency of the backlight control circuit 100 is high and energy waste is low. Thus an important quality factor of the backlight control circuit 100 is high.
The AC voltage includes a normal operation frequency f1, and a startup frequency f2 for lighting up the lamp 130 when the backlight control circuit 100 starts to work. Because the startup frequency f2 is higher than the normal operation frequency f1, the fixed resonant frequency f0 of the RLC resonant circuit can only correspond to one of the normal operation frequency f1 and the startup frequency f2. If the fixed resonant frequency f0 corresponds to the startup frequency f1, the fixed resonant frequency f0 is higher than the normal operation frequency f1. Thus the efficiency of the backlight control circuit 100 is low and energy waste is high. If the fixed resonant frequency f0 corresponds to the normal operation frequency f1, the fixed resonant frequency f0 is lower than the startup frequency f1, Thus each time the lamp 130 is lighted up, flicker is generated in the lamp 130, and the working lifetime of the lamp 130 is reduced by a decrement.
It is desired to provide a new backlight control circuit which can overcome the above-described deficiencies